Technical SBC 350 distributor advance vacuum question

Hi All,

I'm finally getting around to taking a look at the engine on my shoebox ford. It has a SBC 350 with solid lifters (at least it sounded like a sewing machine last time I ran it and listened close to the valve covers).

I noticed there are two breathers on each valve cover, a Holley carb, and no vacuum going to the distributor although it has a distributor advance that is plugged. Should I run vacuum for a car I'll be driving on the street but probably not much (if at all) on the highway)? Anything else I should look into changing (PCV, etc)? Right now the brake booster and th350 trans are the only two items with vacuum lines from what I can tell.

Also I bought some timing tape and am thinking to see what its set to now. Any suggestions on what to look for or what to change? It seemed to run fine last time I drove it just a mile and a half in March. I have not driven it otherwise since I got it.

Thanks,

 

I would say hooking up the vacuum advance on the distributor to manifold vacuum would be a good idea. That will allow advanced timing in high vacuum, low load conditions. I wouldn't expect huge gains, but vacuum advance can help with fuel mileage when you're in cruise mode, and backs timing off when accelerating hard to reduce "pinging" from pre-ignition. Initial timing should be set though with the vacuum line disconnected and the port plugged, but you probably knew that.

Lynn

 

My hotrod has been on the road for 13 years, and almost 15,000 miles without a vacuum hose hooked to the distributor. It runs fantastic, and I don't miss it at all.

 

I'm guessing there is no road draft tube. If there isn't you really should have a PCV setup to properly vent the crankcase.

I'm always amazed at the guys who won't run vacuum advance on a street driven machine. It really does help part throttle fuel economy and has no down side.

 

Different strokes for different folks, that’s what makes the world go around. I’m a firm believer in running vacuum advance.

 

Oh ok, well I see a breather on each valve cover. I'm guessing I need to replace one of them with a fitting that I can run a PCV valve to the carb?

 

Yes, also reverse one valve cover so that the breather holes are not lined up.

 

What does it matter if the valve cover holes are lined up?

 

Valve cover holes misaligned , allow for breathing of the entire crankcase not just front or rear 1/2 . Vacuum advance : one of the best additions to a street driven engine one can do .

Something to think about : if they cost a car manufacture 2.50 each , just think about how many millions in savings in not using it , it wasn’t a profitable addition to operation .

 

You don't get an efficient scavenging of the crankcase if they are in line. In his case, only the front half of the engine breathes. They should be staggered so the whole engine gets drawn through.

 

In addition in some improvement in fuel economy at high vacuum, the extra advance at idle helps the engine run a little cooler when sitting at a traffic light.

 

I have 4 cars tuned for the street currently and none of them run a vacuum advance, including a cool running blown flathead. I may get better mileage but that is not why I built them.

 

For street use, run vacuum advance, period. Use an appropriate PCV valve for your application, adjustable ones are available for hi-po cam applications where oem types don't vent properly. Tune accordingly. Breather placement depends on vehicle use. Always use a baffle under hole in cover. Heavy acceleration use, breathers forward, as oil pools towards rear, road race use, breathers in middle,as heavy brake use pools oil forward and backward, circle track, 2 breathers on left, on basic street car , placement is cosmetic as long as baffle is used, the engine will breathe fine, it doesn't know if breather 6 inches one way or the other.

 

There is no down side.

John

 

Or not.

 

SDrocker- goggle search " On All Cylinders, understanding vacuum advance" by Jeff Smith. A good explanation on the topic for street engines. I have my customers read this to give a basic idea on the concept, and why it is beneficial if they are confused.

 

Don't run the vacuum off the manifold , USE ported vacuum instead
This is connected to the Carb [circled]


This will increase part throttle cruising economy without "part throttle stumble" that sometimes happens.
If you truck idles better with "Manifold" vacuum , It means it hasn't got enough advance
So move it to "ported" and advance the timing a bit more.

From Holley [below]

 

There are lots of things I don't miss, that I would miss if I had used them and then lost them. I'm sure your rod works fine without vacuum advance, and I'm equally sure it could be even better with it.

 

Here is a link to the Jeff Smith article recommended by @Jagmech :
https://www.onallcylinders.com/2015...rstanding-vacuum-advance-and-why-you-need-it/

Those not using vacuum advance should at least take a couple minutes to read this article.

 

I understand they can be tuned to run correctly . What type of distributor is in the Flathead ? I would not recommend running an OEM Flathead Distributor without the vacuum advance connected . If you dig deeper into this you may see that the vacuum advance is the only advance on a OEM Flathead distributor . At least it was on the last Flathead , I tinkered with , which was about 20 years ago , I’m thinking a Y block is the same also .

 

They all run MSD distributers coincidentally. Hey, if something works, why screw with it? The flathead runs a MSD blower timing box to control predetonation. Yeah, that's kinda cheating, no "vacuum advance" but it has electronic advance/retard off a vacuum signal.

 

Ok, at first I said who the hell is Jeff Smith? After reading the article, because I am open minded, I agree and understand what he is talking about. The only thing I didn’t see is, ported or manifold vacuum? Unless I overlooked it, which is better? . I also understand at some point they will be equal. Good article though.

 

Bear in mind that Jeff smith , while by all accounts, is / was a fine fellow , is a writer , AFAIK , not a bona fide automotive electric engineer . If you're going to use someone as a source of wisdom at least choose someone who is unchallengeable , not an influencer ! If you " cruise" a lot ,a vacuum advance , properly set,will probably help ,if you're looking for WOT performance , a vacuum advance is just another possible device that can fail .

 

I like the vacuum connect on a street used engine. I always use venturi vacuum and not manifold.
For the PCV valve mounted in/on a rocker cover, the factory had a separator on the inside without it will suck oil. There is a rubber grommet with a trick vent bottom the Mr Gasket sells I think that works if it clears the rockers…

 

You probably mean ported vacuum, not venturi. They are two different animals. You only get a venturi signal when there is a lot of air running through the venturi in the carb. Holley uses a venturi signal to open the back throttle plates on their vacuum 4 bbls. Ported vacuum is really manifold vacuum with a port just above the throttle plates so there is no vacuum at idle.

 

Chose manifold or ported based on what the engine likes.

That being said, I try manifold first for this reason. The vacuum advance (additional timing) at idle speeds up the engine, allowing the idle speed adjustment on the carb linkage to be lowered.
This saves a miniscule amount of fuel, but more important the leaner idle mixture allows the engine to run a little cooler.
This is important as most rodders who complain about overheating problems, experience them at traffic lights. There are many causes for this overheating, this is one partial solution.

 

You are correct!
As Hillary said over and over, and over, and over.. I misspoke

 

I've got mine at 40⁰ all in. There's 8⁰ in the vacuum advance. If I hook it up, I get spark knock at part throttle-3000rpm. So mine is unhooked. It's a hot rod, I don't expect it to get good mileage lol

 

Lloyd, do you have it hooked to the base plate or the metering block?

 

Damn I opened the worm can ! Sorry